Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector for each cylinder to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder.
Further, engines have been proposed using more than one type of fuel injection. For example, the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. are one example. Specifically, the Heywood et al. papers describe directly injecting ethanol to improve charge cooling effects, while relying on port injected gasoline for providing the majority of combusted fuel over a drive cycle. The ethanol provides increased octane and increased charge cooling due to its higher heat of vaporization compared with gasoline, thereby reducing knock limits on boosting and/or compression ratio. Further, water may be mixed with ethanol and/or used as an alternative to ethanol. The above approaches purport to improve engine fuel economy and increase utilization of renewable fuels.
One issue recognized by the inventor herein with the above approach is that the system may abruptly exhaust a supply of the ethanol, or other knock suppression fuel, and thus abruptly lose the ability to operate at high engine output torque with reduced knock. This can be especially dramatic when the vehicle is operating at high output conditions, such as trailer towing or under mountainous driving conditions. Thus, to avoid significant knocking upon such conditions, the engine must abruptly retard spark and/or reduce boost, thus significantly reducing engine power or torque output.
One approach to address the above issues is to adjust maximum allowable engine power or torque output in response to availability of a knock suppression mixture. For example, the peak torque output of the engine may be gradually reduced (e.g., ramped, etc.) before the knock suppression fluid is depleted so that when the fluid is depleted, the operator does not experience a significant and/or rapid decrease in peak output. In this way, improved driver perception can be achieved.